The yeasts Saccharomyces cerevisiae and Schizosaccharomyces pombe provide excellent model systems for studying cell cycle regulation, because they have fairly typical eukaryotic cell cycles displaying a variety of controls, and because they are easy to analyze by a host of powerful genetic and biochemical techniques. Previous studies have focused on a mitotic control point in S. pombe, and a Gl control point in S. cerevisiae. We believe that each yeast (and eukaryotes in general) actually has a control at both points. Further, we believe the two controls are homologous to each other. We call this the Twin Clock Hypothesis, and our goal is to prove it by finding the components of both clocks in a single system. An additional goal of this project is to study the roles and the regulation of the Cdc2/Cdc28 family of protein kinases.